This application claims Paris Convention priority of DE 102 47 783.3 filed Oct. 14, 2002 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a rheometer with a rotor surrounding a shaft which is seated with play in a recess of a bearing block and is supported contact-free in a radial and/or axial direction by means of an exterior air cushion acting in a bearing section.
Measurement of rheological values in a rheometer is based on the exact knowledge of the relationship between shearing strain or stress and deformation of the substance to be examined. One of the two values is predetermined and the other is measured by a measuring means. To derive the exact material properties from these values, the mechanical friction loss of the measuring means must be kept to a minimum. This is achieved in a rheometer of the mentioned type as described e.g. in U.S. Pat. No. 6,167,752 or DE 44 36 156 C1 in that a shaft of a rotor of the measuring means is supported in a radial and axial direction by air bearings. The air bearing is formed in a bearing body which has a recess with an axial through-hole and a cylindrical chamber extending substantially coaxially thereto. The shaft is inserted into the axial through-hole with play and carries an annular disk which extends perpendicularly to its axial direction and is seated with play in the cylindrical chamber.
The bearing body is penetrated by a web of very fine air channels which terminate on the surface of the through-hole facing the shaft and of the cylindrical chamber and are connected to an air supply. The pressurized air supplied from the air supply flows through the air channels and exits at their openings thereby forming, in the region of the through-hole, an air cushion surrounding and radially centering the shaft, and, in the region of the cylindrical chamber, a further air cushion axially supporting the disk and thereby the shaft.
To produce an air cushion which acts as uniformly as possible over the bearing surface, a plurality of very fine air outlet openings with small mutual separation must be formed and connected to the fine air channels. To achieve the precision required in this case, laser cutting methods are e.g. used, as a result of which the production of the bearing body is very demanding and expensive. Moreover, there is the danger that the heat from the laser beam produces melt-outs or ridges, which can prevent uniform air flow. It has also turned out that the loading capacity, i.e. the bearing force capacity of known air bearings, is limited which suggests that air bearing technology has reached its limits.
It is the underlying purpose of the invention to produce a rheometer of the above-mentioned type with an air bearing which is simple and inexpensive to produce and has a high loading capacity.
This object is achieved in accordance with the invention in a rheometer of the mentioned type in that the wall of the recess in the bearing section is continuous, without bore-holes, and consists of an air-permeable material, wherein the bearing air loads the side of the wall facing away from the shaft and penetrates through the wall.
The invention is based on the underlying idea that the air channels formed through processing of the bearing block, do not terminate in the wall of the recess. The air is guided towards the wall of the recess from the outside only to such an extent that it loads the side of the wall facing away from the shaft. The air cushion which supports the shaft in an axial and/or radial direction is effected in that at least the wall of the recess is formed from an air-permeable material which is preferably sintered carbon, so-called artificial graphite, or air-permeable ceramics or porous plastic material and, in particular, foamed material. The bearing air penetrates through the wall due to its large number of pores or microchannels, thereby leading to a uniform air cushion structure. This produces a high bearing stability for the shaft in a horizontal i.e. radial direction and also in a vertical e.g. axial direction. At the same time, processing required to form the air bearing is simplified, since the bearing air must only be guided to the outer side of the wall by air channels which must not be precisely dimensioned. Very fine bores which penetrate the wall are not needed. This also safely eliminates the above-mentioned danger of producing melt-outs or ridges due to heat.
In a preferred embodiment of the invention, the shaft is borne in its radial direction, wherein the recess has a through-hole which receives the shaft with play and in which it is supported in a radial direction by a radially outer air cushion.
In a preferred embodiment of the invention, the shaft is axially supported by an air bearing in that the recess comprises a cylindrical chamber containing a disk with play which is seated on the shaft and extends substantially perpendicularly to its axial direction. The disk and the shaft are supported in both axial directions by an axial air cushion formed on both sides of the disk.
Pressurized bearing air is supplied to the side of the recess wall facing away from the shaft preferably through several air channels which are formed in the bearing block and are preferably connected to each other. In a preferred embodiment, the side of the recess wall facing away from the shaft or disk, has several annular channels separated in an axial direction of the shaft which distribute the bearing air over the periphery of the shaft thereby ensuring uniform passage or exit of bearing air on the side of the wall facing the shaft. In a preferred embodiment of the invention, the annular channels are connected via several axial channels, separated in the peripheral direction, and are preferably connected to one single common pressurized air source or air supply. The air supply may comprise one or more control valves to produce different air bearing pressures for different regions, i.e. different bearing rigidities within the air bearing.
The air supply for the air bearing formed in the cylindrical chamber which supports the disk and the shaft in its axial direction, preferably also comprises annular channels, one of which is formed on each axial side of the cylindrical chamber at a small separation from its wall such that the air from the annular channels can penetrate through the wall in an axial direction from opposing sides into the cylindrical chamber, wherein the two air flows bear the disk between them and therefore the shaft.
Further details and features of the invention can be extracted from the following description of an embodiment with reference to the drawing.